Field of the Invention
The invention relates to the fields of molecular biology and molecular diagnostics, and more specifically to methods for massively parallel genetic analysis of nucleic acids in single cells.
Description of the Related Art
Certain quantitative genetic analyses of biological tissues and organisms are best performed at the single cell level. However, single cells only contain picograms of genetic material. Conventional methods, such as polymerase chain reaction (PCR), RNA sequencing (Mortazavi et al., 2008 Nature Methods 5:621-8), chromatin immunoprecipitation sequencing (Johnson et al., 2007 Science 316:1497-502), or whole genome sequencing (Lander et al., 2001 Nature 409:860-921), require more genetic material than is found in a single cell and are usually performed with thousands to millions of cells. These techniques provide useful genetic information at the cell population level, but have serious limitations for understanding biology at the single cell level. Current biological tools also lack the capacity to assay genetic measurements in many single cells in parallel.
Conventional single cell techniques are slow, tedious, and limited in the quantity of cells that can be analyzed at once. For example, in pre-implantation genetic diagnosis (PGD), a single cell is removed from a cleavage stage human embryo for genome-wide analysis of genetic diseases (Johnson et al., 2010 Human Reproduction 25:1066-75). Applications such as PGD require time-consuming, hand-guided biopsy technology, and the largest studies include hundreds of single cells. In another example, genetic recombination between loci of interest can be measured in single sperm cells (Jiang et al., 2005 Nucleic Acids Research 33:e91), but a manual analysis of thousands of single sperm would be time-consuming and impractical.
An established method for single cell analysis is fluorescence-activated cell sorting (FACS). Single cells are diluted into reaction wells, and various genetic and molecular biology techniques can be performed in the wells, from whole genome amplification to single locus PCR assays. However, due to the physical limits of parallelization using reaction wells, FACS is only useful for analyzing hundreds of single cells, rather than hundreds of thousands of single cells.
Single cells can also be used as reaction compartments for performing various genetic analyses (Embleton et al., 1992 Nucleic Acids Research 20:3831-37; Hviid, 2002 Clinical Chemistry 48:2115-2123; U.S. Pat. No. 5,830,663). Single cells can be sorted in aqueous-in-oil microdroplet emulsions, and molecular analyses can be performed in the microdroplets (Johnston et al., 1996 Science 271:624-626; Brouzes et al., 2009 PNAS 106:14195-200; Kliss et al., 2008 Anal Chem 80:8975-81; Zeng et al., 2010 Anal Chem 82:3183-90). These single cell assays are limited to single cell PCR in emulsions, or in situ PCR in single fixed and permeabilized cells. Moreover, when analyzing large populations of cells, it is difficult to trace back each gene product to a single cell or subpopulations of cells.
Thus, there is a need for methods for high-throughput, massively parallel genetic characterization of single cells and methods for identifying the cell or subpopulation of cells that originated the genetic material.